1. Field of the Invention
The invention pertains to methods of industrially-scalable ex situ synthesis of graphene, graphene oxide, reduced graphene oxide, and other graphene derivative structures, and nanoparticles and the uses therefor, including but not limited to composites, composite fabrication and coatings, the fields of tribology, nanotechnology, surface finishing, machining and tooling, boring, drilling, tunneling, ballistics, anti-ballistics, heat shielding, heat absorption, lubricant additives, lubricating compositions, coatings, methods of lubrication, methods of polishing hard surfaces, and methods of cutting, drilling, hardening, protecting, and fabricating steel and other hard surfaces. The invention further pertains to the use of abrasive nanoparticles in lubricating compositions for polishing, hardening, protecting, adding longevity to, and lubricating moving and stationary parts in devices and systems, including, but not limited to, engines, turbos, turbines, tracks, races, wheels, bearings, gear systems, and other physical and mechanical systems employing machined interacting hard surfaces, where the abrasive nanoparticles are formed in situ from the lubricating compositions or, in some cases, formed ex situ and then added to lubricants before their use.
2. Description of Related Art
Synthesis of Graphene and Graphene Derivatives.
Single-layer graphene, as a result of its observed and theoretical physical properties, including a large specific surface area, high intrinsic mobility, high Young's modulus (˜1.0 TPa), high thermal conductivity (˜5000 Wm−1K−1), high optical transmittance (˜97.7%), low gas permeability, and high electron transport capacity, has been the subject of considerable study, research, and discussion in recent years (see, for example, Geim, et al., “The Rise of Graphene”, Nat. Mater., Vol. 6, pp. 183-191, 2007; and Zhu et al., “Graphene and Graphene Oxide: Synthesis, Properties, and Applications”, Adv. Mater., Vol. 22, pp. 3906-3924, 2010). Based on these properties of graphene, it has been considered for use in numerous applications such as photocatalysis, energy storage, solar cells, transparent electrodes, semiconductors, high strength/low weight composite materials, protective coatings, and field emission. Large-scale and economical production methods have, however, remained elusive.
Pure graphene is a planar polycyclic single atomic layer of pure carbon in a honeycomb-like lattice of six-membered sp2-hybridized carbon rings. Graphene is theoretically a single pure layer of graphite, although the term graphene is conventionally also used to apply to a material with several stacked atomic layers of graphite or a graphitic layer with minor defects still having material properties similar to pure graphene. Graphene is relatively hydrophobic and is conventionally formed either by exfoliation of graphite, which may be done using supercritical carbon dioxide or by micromechanical cleavage, or by epitaxial growth on silicon carbide or certain metal substrates. Graphene may also be formed in the gas phase by passing liquid droplets of ethanol into argon plasma in an atmospheric-pressure microwave plasma reactor (Dato et al., “Substrate-Free Gas-Phase Synthesis of Graphene Sheets”, Nano Lett., Vol. 8, pp. 2012-2016, 2008).
Graphene nanotube synthesis has also been reported by an aerosol pyrolysis method (Pinault et al., “Carbon nanotubes produced by aerosol pyrolysis: growth mechanisms and post-annealing effects”, Diamond and Related Materials, Vol. 13, pp. 1266-1269, 2004). A solution of 2.5-5 wt % of ferrocene in toluene or cyclohexane was aerosolized with argon and pyrolyzed at 800-850° C. The early stages of carbon nanotube formation were observed. A layer of nanoparticles believed to include iron was first formed on a solid substrate. An ordered carpet of nanotubes grew from this nanoparticle layer, with one nanotube growing from each of the nanoparticles. High-temperature annealing of the samples led to removal of iron from within the nanotubes and improved order in the nanotubes.
Several recent publications have reported the formation of graphene bonds under combustion conditions. In one case, minute quantities of nanoparticles of all four forms of carbon, namely diamond, graphite, fullerene, and amorphous, were detected in a paraffin candle flame (Su et al., “New insight into the soot nanoparticles in a candle flame”, Chem. Commun., Vol. 47, pp. 4700-4702, 2011). In another earlier case, small amounts of nanoparticle graphitic carbon were found upon acid treatment of the soot from a methane flame (Tian et al., “Nanosized Carbon Particles From Natural Gas Soot”, Chem. Mater., Vol. 21, pp. 2803-2809, 2009). In another earlier case, highly graphitic hollow nanotubes were formed from an ethanol flame (Pan et al., “Synthesis and growth mechanism of carbon nanotubes and nanofibers from ethanol flames”, Micron, Vol. 35, pp. 461-468, 2004). Similarly, carbon nanotubes have been synthesized using CO/H2/He/C2H2 gas mixtures burnt with an acetylene flame in the presence of laser-ablated iron or nickel nanoparticle catalysts (Vander Wal et al., “Flame Synthesis of Carbon Nanotubes using Catalyst Particles Prepared by Laser Ablation”, Am. Chem. Soc., Div. Fuel Chem., Vol. 49, pp. 879-880, 2004).
Polycyclic aromatic hydrocarbons (PAHs) form as part of the airborne “soot” contained in the residual particulate matter (PM) of incomplete combustion, pyrolysis, or other low-oxygen thermal degradation of hydrocarbons. As these PAHs are usually deemed undesirable byproducts of imperfect combustion, numerous studies have focused on how to minimize or eliminate altogether the formation of “soot” in combustion processes (see, for example, Coppalle et al., “Experimental and Theoretical Studies on Soot Formation in an Ethylene Jet Flame”, Combust. Sci. and Techn., Vol. 93, pp. 375-386, 1993).
PAHs have a substantially planar structure of fused aromatic carbon rings with hydrogen atoms bound to the peripheral carbon atoms of the matrices. PAHs may be thought of as miniature nanoscopic scales of graphene.
Graphene derivatives include structures having graphitic bonds partially incorporating heteroatoms such as oxygen or other structural imperfections in the carbon lattice. Graphene derivatives, as described herein, also include structures such as nanotubes, nanobuds, fullerenes, nano-peapods, endofullerenes, nano-onions, graphene oxide, lacey carbon, and other non-graphene forms of graphitic carbon which may contain structural or chemical imperfections.
Graphene oxide (GO) is a family of impure oxidized forms of graphene that includes hydroxyl and epoxide groups bonded to various carbon atoms in the lattice matrix. The structural properties of GO have been extensively studied (see Mkhoyal et al., “Atomic and Electronic Structure of Graphene-Oxide”, Nano Lett., Vol. 9, pp. 1058-1063, 2009), yet the exact chemical structure of GO is still the subject of much debate and considerable variability, at least in terms of hydroxyl and epoxide group frequency and location observed in the various samples studied.
GO is also known to include carboxylic acid groups believed to be located at the edges of the carbon sheets. These various functional groups permit further chemical functionalization of GO. Recently, the conversion of carboxyl groups to hydroxyl groups in a graphene derivative has been reported to produce a material that has been called “graphenol”. Various complex and multi-step methods to convert this graphenol to graphene via pyrolysis have been reported, yet these methods include the use of toxic chemicals such as hydrazine (see U.S. Pat. App. Pub. No. 2011/0201739, by Beall, entitled Method and System for Producing Graphene and Graphenol and published on Aug. 18, 2011).
Not unlike graphene, GO is conventionally formed from exfoliated graphite oxide or by oxidation of graphene itself. GO sheets may be purposefully formed in a wide range of oxidation levels with measured oxygen-to-carbon ratios as high as around 1:2. As graphene oxide has its own unique physical and chemical characteristics apart from graphene, its structural variability has made it less attractive for many experimental studies. As opposed to graphene, GO is hydrophilic and an electrical insulator of high stiffness and high strength (see Dreyer et al., “The chemistry of graphene oxide”, Chem. Soc. Rev., Vol. 39, pp. 228-240, 2010).
Graphene oxide was first prepared by the treatment of graphite with potassium chlorate and fuming nitric acid (see Brodie, “On the Atomic Weight of Graphite”, Proc. of the Royal Soc. of London, Vol. 10, p. 249, 1859). A somewhat more efficient process employed sulfuric acid, sodium nitrate, and potassium permanganate to convert graphite to graphene oxide (see Hummers et al., “Preparation of Graphitic Oxide”, J. Am. Chem. Soc., Vol. 80, p. 1339, 1958). Recently, a still more efficient method was reported using sulfuric acid, phosphoric acid, and potassium permanganate (see Marcano et al., “Improved Synthesis of Graphene Oxide”, ASC Nano, Vol. 4, pp. 4806-4814, 2010).
Colloidally-dispersed GO in water may be chemically reduced to graphene using hydrazine monohydrate. Other chemical reductants for GO include hydroquinone, gaseous hydrogen, and strongly basic solutions. Thermal exfoliation and reduction of GO occurs upon heating to 1050° C. with extrusion to remove the generated byproduct of carbon dioxide gas. Finally, electrochemical reduction of GO may be accomplished by placing electrodes at opposite ends of a GO film on a non-conductive substrate, followed by the application of an electrical current to the film.
Although to date a complete reduction of GO to graphene has not been reported in the literature, GO may be reduced by a number of different processes to produce so-called “rGO” (reduced graphene oxide) with measured oxygen-to-carbon ratios as low as about 1:24.
It is noteworthy that rGO has been observed to exhibit many chemical, physical, and electrical properties more similar to those of graphene than to those of GO.
Graphene and its many derivatives are currently the subject of numerous studies and widespread research, in part because of their many potential applications, including but not limited to lubricants, molecular level coatings for composite reinforcement, heat shielding, ballistic transistors, integrated circuits and reinforced fibers and cables.
Use of Sequestered Waste Carbon in Graphene Production.
Various forms of carbon waste sequestration are known to the art, including, but not limited to, the conversion of carbonaceous wastes to things like “biochar” or synthetic methanol from carbon dioxide (see, for example, Hogan et al., “Biochar: Concept to Sequester Carbon”, Encyclopedia of Earth, National Council for Science and the Environment, Washington, D.C., 2011; Jiang et al., “Turning carbon dioxide into fuel”, Phil. Trans. R. Soc. A, Vol. 368, pp. 3343-3364, 2010), yet the beneficial use of such sequestered or captured carbon wastes as carbonaceous feedstock or promoters in the synthesis of graphene remains unreported.
Implantable Medical Prosthetic Devices.
An important factor in the success of implanted medical prostheses is the uniformity of the friction surfaces, both for longevity and prevention of infection purposes. Asperities on the surface of medically implanted metallic devices provide a location to harbor bacteria. For metallic devices in the vasculature or circulatory system, they also provide a location for dangerous platelet aggregation that can lead to heart attack or stroke. The nanopolishing of such implantable medical prosthetic devices to near atomic-level perfect smoothness would greatly advance the safety and efficacy of such devices.
Nano-Pharmaceuticals, Oncology, and Medical Imaging.
Improvements in the targeting of radiation or chemotherapeutic drugs to a cancer site and the ability to provide contrast for medical imaging are areas of active research in the medical field. Magnetite nanoparticles have been used as a tumor contrast agent for magnetic resonance imaging (see, for example, Tiefenauer et al., “In vivo evaluation of magnetite nanoparticles for use as a tumor contrast agent in MRI”, Magnetic Resonance Imaging, vol. 14, no. 4, pp. 391-402, 1996). There is considerable current study and research into the use of “functionalized” buckyballs as a means to deliver targeted drug therapies to tumors in the body (see, for example, Yoon et al., “Targeted medication delivery using magnetic nanostructures”, J. Phys.: Condens. Matter, vol. 19, 9 pages, 2007).
Steel Production.
Pits and asperities on the surface of steel provide a surface for the formation of destructive oxidation in the form of ferric oxide, also known as rust. The reduction or elimination of these pits and asperities would increase the longevity of such steel structures.
Graphene and GO Reaction Environments.
In some embodiments of the invention, graphene and graphene oxide structures are used in various solvents to act as reaction envelopes, which create a nano-environment for reactions to occur that are thermodynamically or otherwise unfavorable similar to the way enzymes work in biological systems. These graphene reaction envelopes (GREs) and graphene oxide reaction envelopes (GOREs) permit chemical reactions and atomic reformations, such as restructuring atoms into crystals, to occur which would not normally occur outside the reaction envelope. The GRE or GORE serves as a “micro- or nano-reaction vessel” and then may pinch off part of the envelope into a nanoabrasive or other nanoparticle, thereby becoming part of the reaction product. In one embodiment, the envelope acts as a nano-blast furnace for the production of nano-steel from iron.
Steel may take on a number of different forms, including, but not limited to ferrite, austenite, pearlite, martensite, bainite, ledeburite, cementite, beta ferrite, hexaferrum, and any combination of these, depending on the conditions under which it is made. Nano-steels of the invention formed in GREs or GOREs may be in the form of ferrite, austenite, pearlite, martensite, bainite, ledeburite, cementite, beta ferrite, hexaferrum, and any combination of these.
Nanosteel, Nanorobotics and Nanomachine Fabrication.
Nano-crystalline metallic alloy synthesis is known in the art (Alavi et al., “Alkaline-Earth Metal Carbonate, Hydroxide and Oxide Nano-Crystals Synthesis Methods, Size and Morphologies Consideration”, pp. 237-262 in Nanocrystals, ed. by Matsuda, InTech, Rijeka, Croatia, 2011). Synthesis of steel-reinforced nanoparticles, nano-onions, and methods of producing neat nano-steel crystals and nanoscopic metal sheets, however, remain unreported.
Nanorobotics commonly refers to the science of nanotechnology engineering and fabrication of mechanical devices in the range of 0.1 to 10 μm in size from nanoscale components. Other common names for these theoretical devices are nanobots, nanoids, nanites and nanomites. It is postulated that future developments in this field will allow the construction of, among other things, tiny remotely operated surgical instrumentation and nanoscale electronic devices. Easy and inexpensive methods of nano-fabrication of tiny steel crystals or billets would likely advance this science considerably.
Concreting and Asphalting Technology.
Concrete and asphalt concrete are two common composite materials used in construction. Concrete is a composite formed minimally of a cementitous material, a fine aggregate, a coarse aggregate, and water. Asphalt concrete is a composite typically formed minimally of asphalt, a highly viscous, sticky black tar-like substance present in some crude petroleums and natural deposits, and a coarse aggregate. Many types of admixtures and additives have been developed over the years in an attempt to increase the strength of these materials.
The most pervasive of these concrete “additives” fall into two general categories: water-reducing superplasticizers (also known as high-range water reducers) and synthetic reinforcing fibers used to produce fiber-reinforced concrete (FRC). The superplasticizers, including the latest generation of polycarboxylate ether based superplasticizers (PCEs) and polypropyleneglycol-derivative admixtures, serve to reduce the amount of water required to form the composite. Superplasticizers also improve the rheology (flow characteristics) of the concrete slurry, thereby improving workability prior to cure (see Palacios et al., “Effect of superplasticizer and shrinkage-reducing admixtures on alkali-activated slag pastes and mortars”, Cement and Concrete Research, Vol. 35, pp. 1358-1367, 2004; Aitcin et al., “Superplasticizers: How they Work and Why The Occasionally Don't”, Concrete International, Vol. 16, pp. 45-52, 1994).
In the case of FRC, the synthetic fibers (typically polypropylene fibers), are intended to increase the strength of the matrix and improve the concrete's deformability. Concrete reinforcing fibers are meant to bridge micro-cracks in concrete and reduce separation, thereby allowing the concrete to maintain its ability to support its load without failure from complete separation along cracks (see Soroushian et al., “Mechanical Properties of Concrete Materials Reinforced With Polyproplene or Polyethlene Fibers”, Materials Journal, Vol. 89, pp. 535-540, 1992).
In practice, neither type of “additive” has shown dramatic increases in the strength of the concrete or asphalt concrete products or systems. It is believed that graphene and certain graphene derivatives could be used as reinforcing “additives” to concrete and asphalt in lieu of the methods of the current state of the art.
Military and Ballistics Science.
According to recent research at Columbia University (New York, N.Y., United States), graphene is identified as the strongest material on Earth. The inordinate strength of graphene is attributed by the Columbia researchers to its covalent carbon-carbon bond matrix. The graphene samples tested were defect-free monolayers of graphene. Testing of the samples revealed that a single sheet of graphene has an intrinsic strength of 42 Nm−1.
Modern anti-ballistics science seeks to develop ever-increasing thinner means of providing protection from ballistic projectiles and shrapnel. Towards this end, new means for molecular reinforcement of polymer-matrix-composites (PMCs) are continually being investigated. The current state of the art employs several varieties of high-performance ballistic yarns and fibers, including S-glass, aramids (e.g., Kevlar® 29, Kevlar® 49, Kevlar® 129, Kevlar® KM2, Twaron®), highly oriented ultra high molecular weight polyethylene (e.g., Dyneema®, Spectra®), PBO (e.g., Zylon®) and Polypyridobisimidazole (PIPD) (referred to as M5®) etc.
Typical characteristics of these fibers are very low density and high tensile strength, with correspondingly high energy absorption capacity. In the case of polymer matrix composite (PMC) ballistic panels, the fibers' force-dispersing deformation ability is severely hampered by the surrounding resin of the composite, which leads to failure under conditions of fracture and delamination of the resin matrix upon impact from a projectile. Graphene and its derivatives, incorporated into textile composite ballistic panels, would not suffer from the same limitations as typical PMC resin-matrices.
Graphene and its derivative structures represent a unique opportunity and material for anti-ballistics. Graphene and its derivatives have particularly high elastic moduli and tensile strength, with a Young's modulus of ˜1000 Gpa and a strength of around 13-53 Gpa. In comparison to traditional anti-ballistic fibers and composites, the potential of graphene and its derivatives far outshine the methods of the current state of the art.
A company known as Nanocomp Technologies Inc. (Concord, N.H., United States), working in conjunction with the U.S. Army's Natick Soldier Center, is seeking to develop a new generation of lightweight ballistic armor based on carbon nanotube (CNT) technology. In April of 2009, the company reportedly demonstrated that a ˜5 mm thick CNT-composite ballistic panel was able to stop a 9 mm bullet. Additional advancements in industrially-scalable graphene and graphene-derivatives synthesis would undoubtedly move this technology closer to commercialization.
Lubrication of Mechanical Systems.
All mechanical systems involve friction between interacting constituent parts. Such interaction can be as simple as a ball bearing sliding along a race, a piston ring moving against a cylinder sleeve, or the contact between the lobe of a camshaft and its cam follower. In all of these examples, friction between the interacting surfaces is a factor to be considered. Friction in any system is the cause of stress, fatigue, wear, heat, noise, vibration, and eventually failure. The other common enemy of the aforementioned metal-containing mechanical systems is corrosion.
In most circumstances, engineering science seeks to reduce the friction inherent in physical and mechanical systems with interacting surfaces by machining and finishing those surfaces to the highest practical smoothness. No current friction surface is perfectly smooth, that is to say, completely free of asperities. Interaction of these uneven surface asperities increases friction. As needed, interacting components of physical and mechanical systems are machined and polished to required tolerances to permit proper performance and reduce inherent friction. Marked reduction in friction through so-called “super-polishing” of components to high tolerances (Ra<50 nm) to date has meant substantial additional production time and costs. Generally, modern machining science is forced to trade machining exactitude for economy.
Additionally, all internal combustion engines, including gasoline and diesel, both normally aspirated and turbocharged, turbines, and other gear-containing systems require lubrication for proper operation. Various attempts at providing optimum lubrication of these machines have been made in the field of engine and gear lubrication since their inception. The first such attempts at lubrications, such as olive oil and certain carbolic soaps, have since been replaced with more sophisticated hydrocarbon-based lubricants, many containing even more sophisticated additive packages and each such additive attempting to address various inherent problems in lubricating these systems.
The current state of the art for lubrication of metal-containing mechanical systems, such as internal combustion engines, is the use of elasto-hydrodynamic lubrication (EHL) techniques that utilize methods and materials to “deal with” the problem of asperities on interacting metallic surfaces of mechanical systems by employing incompressible fluids and barrier coatings to prevent metal-to-metal contact. None of these methods affect the so-called Ra (Roughness Average) values of the interacting metallic component surfaces and do nothing to ameliorate the friction-causing effects of the asperities themselves.
To preserve and protect metal friction surfaces and the systems that include them, various lubricant additives are used for a variety of purposes, such as dispersants, corrosion inhibitors, viscosity improvers, seal swell agents, pour point depressants, foam inhibitors, anti-wear agents, and antioxidants. Some lubricant additives that have been developed to reduce friction include the following: triorthocresyl phosphate (TOCP, or simply TCP), popular in aviation lubricants but known to slowly attack elastomer gaskets and seals; naphthenic hydrocarbon detergents, known to combine with the products of incomplete combustion to form hydrochloric acid; zinc dialkydithiophosphates (ZDDPs), problematic to vehicles equipped with catalytic converters; chlorinated paraffins, identified globally as extremely harmful to aquatic life; suspended solids such as polytetrafluoroethylene (PTFE, trade name Teflon®), considered undesirable for lubrication by many; graphite powder, considered by many to be undesirable in systems employing bearings; molybdenum, a metal reported to reduce fuel economy; tungsten disulphide nano-onions, a temporary barrier solution; Buckminsterfullerenes, another expensive and temporary solution, and nanodiamonds suspended in graphite to discourage typical aggregation of the abrasive particles, again invoking complaints by those who object to graphite employed in systems containing bearings. Lubricant additives often also contain phosphates and sulfides that upon decomposition, can contribute to the production of noxious gases.
Carbonaceous deposits within mechanical systems are almost universally considered undesirable, so many modern lubricants are specifically designed and formulated to inhibit and/or prevent the formation of any carbonaceous deposits. Conventional EHL wisdom suggests that internal combustion engine oil lubricants must be formulated to be as physically and chemically stable as possible to resist thermal degradation of the base lubricant and its additives by incomplete combustion and pyrolysis, not encourage it; this because the products of such thermal breakdown of conventional lubricants produce harmful carbonaceous deposits (such as sludge) that tend to clog valves, coat piston rings and generally decrease the operating efficiency and life expectancy of an engine. Dispersants are commonly used in lubricants to prevent the aggregation of sludge (see, for example, Won et al., “Effect of Temperature on Carbon-Black Agglomeration in Hydrocarbon Liquid with Adsorbed Dispersant”, Langmuir, Vol. 21, pp. 924-932, 2005; Tomlinson et al., “Adsorption Properties of Succinimide Dispersants on Carbonaceous Substrates”, Carbon, Vol. 38, pp. 13-28, 2000; Wang, “Synthetic and Characterization of Ethylene Carbonate Modified Polyisobutylene Succinimide Dispersants”, University of Waterloo Masters Thesis, 2010). Blow-by soot from the engine that is the result of incomplete combustion has been shown to be highly abrasive and capable of damaging metal parts (see, for example, Jao et al., “Soot Characterisation and Diesel Engine Wear”, Lubrication Science, Vol. 16, pp. 111-126, 2004; Ryason et al., “Polishing Wear by Soot”, Wear, Vol. 137, pp. 15-24, 1990; Yamaguchi et al., “Soot Wear in Diesel Engines”, Journal of Engineering Tribology, Vol. 220, pp. 463-469, 2006; Gautam et al., “Effect of Diesel Soot Contaminated Oil on Engine Wear—Investigation of Novel Oil Formulations”, Tribology International, Vol. 32, pp. 687-699, 1999). “Ashless” engine oils are another example of products supporting the notion that lubricating formulations must be kept as free of carbon particles as possible and that all carbonaceous engine deposits are harmful and bad. Under the current EHL paradigm, thermal degradation and pyrolysis of lubricant additives resulting in the formation of carbonaceous soot and deposits is universally deemed undesirable.
The current testing standards for lubricants and their additives are further evidence of, and support this, lubrication paradigm. The Noack Volatility Test (ASTM D5800) measures vaporization of the lubricant formulation as a function of temperature, because formulations become more viscous with increased vaporization. The test involves putting a mass of motor oil into a Noack device at 250° C. with a constant flow of air over the sample for 1 hour. Then the sample then is weighed to determine loss of mass due to loss of volatile organic compounds (VOCs). The acceptable loss in mass is to be no greater than ˜13 to 15%. A lubricant must pass this test to earn approval under the API CJ-4 motor oil standard (United States) or the ISLAC GF-4 motor oil standard (European Union).
Other lubricant industry evaporation tests include ASTM D972 and ASTM D2595. ASTM D972 tests the lubricant formulation at temperatures between 100 and 150° C. with a constant flow of air (2 L/min) over the sample. ASTM D2595 tests the lubricant formulation at temperatures between 93 and 316° C. with a constant flow of air (2 L/min) over the sample.
The modern lubricant industry almost exclusively uses a base lubricant of linear or branched chain hydrocarbons in EHL lubricating formulations, along with relatively small amounts of a combination of comparatively expensive additives, including, in some cases, cyclic-carbon containing additives such as certain antioxidant “hindered” phenols, certain salicylates, and certain amines. Most often, use of cyclic-carbon containing lubricant antioxidant additives in the prior art is limited to efforts to improve or protect the underlying base lubricant mostly by inhibiting its oxidation from the radicals of in situ formed peroxides.
The entire aforementioned EHL lubrication paradigm and the industry testing standards are based upon the premise that carbonaceous products of incomplete combustion or pyrolysis are universally harmful and undesirable inside of engines and mechanical systems. This suggests that the optimal result of use of lubricants containing detergents, dispersants, and boundary films is to maintain the lubricated internals of a mechanical system as perfectly clean and free of carbonaceous deposits and free of abrasive wear as possible.
Production and Use of Fullerenes in Lubrication.
Fullerenes, first discovered in 1985 and named after the late geodesic dome architect Buckminster Fuller, are a class of molecules with outer shells composed entirely of carbon rings. The basic spherical variety of fullerene is buckminsterfullerene or simply, a “buckyball”. Buckyballs can be endohedral in nature, with various atoms, ions, or complexes trapped inside their hollow cores. Endohedral metallofullerenes, which contain metallic ions, are the subject of significant current scientific inquiry and study.
In mathematical terms, a buckyball is a trivalent convex polyhedron comprised of pentagonal and hexagonal carbon rings. Buckyballs follow Euler's polyhedron formula, in that V−E+F=2, where V, E, and F are the number of vertices, edges, and faces on the exterior of the ball. In terms of the non-isomorphic fullerenes, there are some 214,127,713 different varieties. Pure simple buckyballs are commercially available in C60 and C70 configurations, but are quite expensive; generally, $900 to $1,000 per 100 mg of material.
Bucky-diamonds are nanoscale carbon complexes of a diamond core within a fullerene or fullerene-like outer shell (see, for example, Barnard et al., “Coexistence of Bucky-diamond with nanodiamond and fullerene carbon phases”, Physical Review B, Vol. 68, 073406, 2003). This structure is now believed to be an intermediary structure between the interconversion of nano-onions and nanodiamonds. Barnard et al. predict Bucky-diamonds to be a metastable form of carbon as the coexistence of nanodiamond and fullerene in a size range of ˜500 to 1,850 atoms (˜1.4 to 2.2 nm in diameter).
Barnard et al. (“Substitutional Nitrogen in Nanodiamond and Bucky-Diamond Particles”, J. Phys. Chem. B, Vol. 109, pp. 17107-17112, 2005) present that it is possible to incorporate heteroatoms, such as nitrogen in this case, into the Bucky-diamond structure. Recently, Yu et al. (“Is There a Stable Bucky-diamond Structure for SiC Cluster”, submitted to the Journal of Chemical Physics on Aug. 24, 2011) proposed a stable Si68C79 Bucky-diamond structure based on computer molecular modeling. In the stable state, the nanodiamond core and the fullerene-like shell are not believed to be chemically bonded to each other. The Yu et al. modeling predicted that upon heating of this Si—C structure, the 35-atom core would decompose at a lower temperature than the 112-atom shell, the core then becoming incorporated into the shell to form a larger retained fullerene-like shell structure upon cooling.
Fullerenes are a promising new nanotechnology in lubrication science. There have been many attempts to use fullerenes as barrier lubricants to fill asperities and provide a tribological film on moving parts. Unfortunately, large-scale and commercially-viable means for the production of useful fullerenes has proven elusive. Again, the current state of the art in tribology has focused on tribological films and coatings on the surfaces of moving parts. However this old paradigm does not address the underlying cause of friction itself, the asperities on the interacting metal parts.
The advent of nanotechnology and the science of tribology have introduced several new approaches to lubrication through the use of various nanoparticles. U.S. Patent Application Publication No. 2007/0292698, entitled “Trimetaspheres as Dry Lubricants, Wet Lubricants, Lubricant Additives, Lubricant Coatings, Corrosion-Resistant Coatings and Thermally-Conductive Materials” by Gause and published Dec. 20, 2007, discloses the use of scandium-containing metallofullerene buckyballs as a suspended solid lubricant, in place of simple carbon fullerenes or “buckyballs”, which rapidly degrade at elevated temperature.
The use of externally separated singular nano-Buckydiamonds (SNBDs) as lubricant additives has been postulated, however these molecules are inherently difficult to separate from undesired agglomerates, a necessary step to make them useful in lubrication and other applications (See for example, Ho, D. (ed.), Nanodiamonds: Applications in Biology and Nanoscale Medicine, Ch. 1, “Single-Nano Buckydiamond Particles, Synthesis Strategies, Characterization Methodologies and Emerging Applications”, by Ōsawa, E., Springer Science+Business Media, LLC, New York, 2010).
NanoMaterials, Ltd. (Nes Ziona, Israel) has produced a series of tungsten disulfide nanopowder-containing lubricants. These black tungsten sulfide onion structures are intended to fill surface asperities and shed layers to act as a low-friction interaction barrier surface between interacting metal engine components.
NanoLube, Inc. (Lombard, Ill., United States) claims to produce non-abrasive carbon nanospheres, under the name DiamondLube™, that are introduced into lubricants to reduce friction. The NanoLube™ product appears to be expensive but simple fullerenes suspended in lightweight oil.
PlasmaChem GmbH (Berlin, Germany) markets an additive for motor oils under the trademark ADDO®, which is claimed to contain diamond and graphite nanoparticles formed by detonation synthesis capable of polishing internal engine parts to mirror-like smoothness. The graphite is presumably added to the suspension to reduce agglomeration of the nanodiamonds.
Detonation nanodiamond is a nanodiamond product typically formed by explosive detonation of an oxygen-deficient mixture of trinitrotoluene and hexogen (see, for example, Mochalin et al., “The properties and applications of nanodiamonds”, Nature Nanotechnology, Vol. 7, pp. 11-23, 2012). The resulting nanodiamonds are usually in the form of 1-1 μm clusters of 5 nm diamondoid particles, each nanoparticle comprising a diamond core with a layer of surface functional groups.
Other methods of forming nanodiamonds use non-detonation techniques, such as laser ablation, high-energy ball milling of diamond microcrystals, plasma-assisted chemical vapor deposition, autoclave synthesis, chlorination of carbides, ion irradiation of graphite, electron irradiation of carbon nano-onions, and ultrasound cavitation. These resulting non-detonation nanodiamonds have a tendency to cluster upon synthesis, and much effort has been devoted to developing processes to cleanly separate the agglomerated nanodiamond products.
The common element in the majority of these solutions to friction reduction, as well as the current state of the art, is the use of elasto-hydrodynamic lubrication (EHL) techniques that utilize methods and materials to “deal with” the problem of asperities on interacting metallic surfaces of mechanical systems, not to remove or “solve” the root cause of the problem—the asperities themselves. Those methods and materials that do attempt to address polishing and reduction of asperities do so by employing externally added nanodiamond abrasives that must be suspended in materials employed to prevent their agglomeration into undesirably large clusters. None of the aforementioned methods and materials involve techniques or the means for in situ formation of beneficial carbonaceous tribological particles or nano-abrasives from liquid precursors, a novel approach that addresses the inherent problem in the current state of the art of undesired particle agglomeration from externally-added nanodiamond lubricant abrasives.